1. Field of the invention
The present invention relates to a friction agitation jointing method of metal workpieces used in various fields of transportation apparatus and heavy and light electric machines.
2. Description of the Related Art
In recent times, a jointing method referred to as friction agitation jointing method has been reduced into practice. According to the friction agitation jointing method, workpieces are jointed together by solid phase bonding in which a rotor (1) as shown by FIG. 8 is used as a tool. According to the rotor (1), a front end axial core portion of a cylindrical rotor main body (2) is coaxially and integrally projected with a friction agitation probe (3) in a pin-like shape having a diameter smaller than that of the cylindrical rotor main body (2) and the rotor (1) is fabricated by a material such as steel which is hard and excellent in heat resistance. Further, although illustration is omitted, recesses and protrusions for agitation are formed on the surface of the probe (3).
In jointing operation, while the rotor (1) is being rotated around its own axis, the front end of the probe (3) is brought into contact with an abutment boundary portion (53) between workpieces (51) and (52) in a pressed state and the contact portion is softened and plasticized by the frictional heat. Further, the rotor (1) is pressed further to the workpieces (51) and (52), the probe (3) is embedded in the wall thickness direction of the workpieces (51) and (52) and a shoulder portion (4) at the front end of the cylindrical rotor main body (2) is brought into contact with the workpieces (51) and (52) in a pressed state. Thereafter, while maintaining the state, the rotor (1) is moved along the abutment boundary portion (53) between the workpieces (51) and (52). At the abutment boundary portion where the rotor (1) passes, the surrounding material is softened and agitated by the frictional heat caused by rotation of the rotor (1), further, after the material has been plastically flowed to embed a groove produced by passing the probe (3) while scattering of the material is being restricted by the shoulder (4) of the cylindrical rotor main body (2), the material loses rapidly the heat and cooled and solidified. In this way, softening, adhesion, deformation, agitation, cooling and solidification of the material at the abutment portion (53) are successively repeated in accordance with movement of the rotor (1) and the workpieces (51) and (52) are integrated together at the abutment portion (53) and are successively jointed (58).
The friction agitation jointing method is considerably expected to achieve power in jointing metal workpieces in view of a point that the workpieces (51) and (52) are jointed together in a softened state without melting the material and the joint portion is difficult to undergo metallurgical effect caused by a heat affected zone as in welding.
However, according to the friction agitation jointing method, there poses the following problem when metal workpieces having different deformation resistance and metal workpieces having different melting points are jointed.
That is, in the case of jointing together the metal workpieces having different deformation resistance, when jointing operation is carried out by exerting an abutment portion with an agitation force necessary for sufficiently plastically flowing a workpiece having larger deformation resistance, plastic flow of the workpieces having the larger deformation resistance becomes dominant at the abutment portion and there is caused a drawback in which a uniform agitation jointing state of members for use cannot be achieved. Meanwhile, when jointing operation is carried out by exerting small agitation force to the abutment portions, although a workpiece having smaller deformation resistance is plastically flowed, the plastic flow of a workpiece having larger deformation resistance becomes insufficient and there causes a drawback in which excellent jointing state cannot also be achieved.
Meanwhile, in the case of jointing together metal workpieces having different melting points, when jointing operation is carried out by elevating temperature of the joint portion to softening temperature of a workpiece having higher melting point, melting is caused in a workpiece having lower melting point at the temperature and solid phase bonding is not established and there causes a drawback in which a metallurgical effect caused by a heat affected zone of the joint portion is resulted. Meanwhile, when jointing operation is carried out by elevating temperature of the joint portion to softening temperature of a workpiece having lower melting point, softening of a workpiece having higher melting point becomes insufficient, agitation of materials is not carried out excellently and there causes a drawback in which firm bonding cannot be achieved.